Automatic teller machines (ATM) are now common. Each machine carries a number of removable cassettes in which money is stored. Stamps or tickets may be stored in similar types of machines. It is necessary, from time to time, to replenish the cassettes in the ATM. This is normally done at fixed intervals based on the expected usage of the machine. A security vehicle and guards is dispatches from a central location to take full cassettes to an ATM. The cassettes may typically contain £200,000 and consequently theft of one or more cassettes represents a relatively easy way of obtaining a significant amount of money for a criminal. Furthermore, when a new cassette is installed in a machine, a “empty” cassette is removed from the machine. However, since the replenishment is done at predetermined intervals, rather than when the cassettes are low or empty, the “empty” cassettes may in fact contain a considerable amount of cash. Some £3.5 million was lost in 1995 in the UK alone through robberies on ATM machines when money was being transferred.